24/03/04 11:07:14 AM
G1176 Gain Mods
I made this its own topic because I think a lot of people (well at least 3 or 4 had less gain than they wanted/expected.
I looked at some other 1176 schematics and this one http://www.waltzingbear.com/Schematics/Urei/Urei_1176LN_series_F_page_2.htm had a bit of a different line amp. There are some different values of resistors and one different cap around the BC107 in the line amp.
I used the new values and left the 4.3K in since it was close to the 3.9k and probably yeilds a little less feedback?
Anyhoo, It got LOUDER. I don't know much about the whole db thing but meter wise it seems to have gained 4db. Its definately an audible difference and it was pretty easy to change out the parts. Theres only five of them.
Bluebird
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I modded all of our G1176s by re-jigging the way that the o/p tx is connected up.
Download the spec sheet from Lundahl; it'll make this easier.
Connect pins 2 and 4 and use these as the i/p. Connect pins 5 and 1 to 3 and make sure these are grounded. On the secondary side, connect pins 6 and 9 then use pins 10 and 7 as + and - o/p.
This gives something like 35dB of gain or there abouts...
I was also getting a load of 100Hz hash on the audio gnd so I isolated the p/s gnd and Starred it at the chassis. This made it very quiet!
Chef
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Chef,
You mean you measured 35dB of gain with unit bypassed and input and aoutput pots fully CW?
I'm actually at 30,5dB with these settings and no mods done, can anyone please measure and post their gain?
Frank
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If I remember correctly I had 1KHz going in at -40dB, the attack set to Off (Fully counter), and the In and Out controls set to max.
If you are getting that with out the mods, great! I wasn't and could not find any component errors so I thought laterally. No body could tell me what gain they were getting and I was only getting around 20dB, so I just did what I had to to get it to work. There may well be a huge error that I've made but at least I've got a useable box...
FYI the Purple has around 43dB...
Chef
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Chef,
The manual for the original unit says it has 45dB so I wish to go at least 6dB up from my 30,5. Do you also have a big difference in level from bypass to gr mode even with input set low?
Does your transformer mod give impedance problems?
Many thanks,
Frank
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Wes, I don't have any impedance problems. Our 1176's have been road tested in 2 different studios, and in here on the bench and there do not seem to be any complaints. I don't know if it bears any relation to the total amount of gain, but what rev. manual do you have? the total gain may change with some of the revisions. Note: that is a GUESS!!!!!! but may be the case.
As the amount of comp is directly related to the position of the I/P pot I imagine that if you have both I and O pots on max and switch in the compression it will almost certainly begin to compress...?
As for just switching comp in and no comp occuring, no there is no drop in level, but it is so dependent on I/P level and the pos of the I/P pot...errr
p.s. Wes, are you sure you really need 43dB? No one has complained about the 35db we have and that's in a £1000 per day studio...Just a thought.
Chef
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Hello Frank,
Here are the results of the gain analysis of my G1176, as well as details of specific test parameters:
input XLR to output XLR gain = 29.9db
signal preamp gain(input XLR to high side of output pot) = 25.5db
signal line amp gain(high side of output pot to output XLR) = 4.4db
gr control amp gain[input of gr control amp (point 22) to differential output of C19 and C20] = 24.9db
The details of my testing procedure are as follows:
input pot = maximum
outpot pot = maximum
attack = full ccw
release = full cw
ratio switch = 20:1
meter switch = bypass(switched to gr for one phase of test)
input = 40mV RMS @ 1kHz at input XLR(pins 2&3, balanced connection)
output = 1.25V RMS across XLR pins 2&3
high side of input pot to ground = 40mV RMS
high side of output pot to ground = 750 mV RMS
juncture of R40 and R41 to ground = 1.2V RMS
pin 1 of 5402 opt to ground = 14mV? (could not get reading with DMM or scope!?!?)
With everything the same, except meter switch in gr position:
output = 1.05V RMS = 1.5db gain reduction(VU meter shows -1.75db)
high side of output pot to ground = 635mV RMS
point 22(input of gr control amp) = 320mV RMS
differential output of gr control amp(C19 and C20) = 5.6V RMS
DMM used for AC voltage measurments has 10M input impedance. It measures accurately at 1kHz(the manual states flat response up to 400Hz, but the difference at 1kHz is only about a millivolt).
Semiconductors substituted:
NTE 129p for BD518 (Q9)
NTE 128p for BD517 (Q
All static DC voltages check good per Jakob's measurements.
No input transformer.
No mods.
A quick synopsis of my exact Q-bias trimming procedure:
input = 50mv RMS @ 1kHz at input XLR(pins 2&3, balanced connection)
input pot = 12:00
Q-bias trimpot = full cw
output = 112mV RMS at output XLR(pins 2&3)
meter switch = bypass
ratio = 20:1
attack = full ccw
release = full cw
output pot = 2:00
I turned the Q-bias trimpot ccw until the output reading dropped to 100mV RMS(approximately 1db drop). This left the trimpot at about 2:00
Let me know if this data throws up any red flags or if you need any other information or measurments from my G1176.
Dean
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